1. Field of the Invention
The present invention relates to a lithium niobate (LiNbO.sub.3, which will hereinafter be referred to simply as LN) single crystal which is useful in the field of e.g. optical information processing, photo-processing, photochemical reaction or photometric control, utilizing a laser beam. More particularly, it relates to a LN single crystal for an optical element having improved hologram diffraction efficiency and response speed, a method for heat treatment thereof and a hologram-application element containing the LN single crystal.
2. Discussion of Background
When a LN single crystal is used for an optical application, if a strong laser beam is irradiated thereto, a local change in refractive index (a photo-refractive effect) appears, and an attention has been drawn to an application in which this photo-refractive effect is positively utilized and applied to a phase-type hologram recording element as a highly sensitive optical memory. With respect to the photo-refractive effect, its mechanism has been explained as follows. Namely, the photo-refractive effect is a phenomenon which occurs when light is irradiated to an electro-optic crystal having a deep trap level (i.e. a photo-refractive center) attributable to an impurity or defect. The photo-refractive center is photoionized, and free carriers thereby formed are diffused and moved by e.g. the external electric field or the photovoltanic effect and then reunited at a non-irradiated portion, whereby a space-charge distribution corresponding to the light intensity distribution will be formed. This space-charge distribution will bring about the change in refractive index by the electrooptical effect. This is the photo-refractive effect.
In order for the above mechanism to function, there must be an ionized trap level at the initial state. For examples, in a LN single crystal, Fe.sup.2+ forms a doner level, which is ionized to Fe.sup.3+ thus forming a trap level. Accordingly, it has been known that an impurity, particularly a transition metal impurity, contained in the crystal plays an important role for the photo-refractive effect. Especially, a LN single crystal is relatively inexpensive among many electro-optic crystals and a large size crystal is readily available. Besides, it has an excellent characteristic as compared with other photo-refractive crystals, such that the recording time of hologram lasts at least a few months. Accordingly, it is used for a research for an application to a memory element for phase type hologram recording.
On the other hand, a LN single crystal with a congruent composition having no transition metal incorporated, has an extremely low hologram diffraction efficiency and thus is not suitable for such an application. To improve the hologram diffraction efficiency of a LN single crystal, it has been heretofore common to employ a crystal having a few hundred ppm of Fe incorporated. In such a case, the composition of the mother crystal is a congruent composition. The reason for employing this composition is that it has been believed that in order to produce a LN single crystal having a high uniformity of composition, it is essential to grow the crystal by a rotational withdrawing method from a melt having a congruent composition with a molar fraction of Li.sub.2 O/(Nb.sub.2 O.sub.5 +Li.sub.2 O) of 0.485, where the crystal and melt are coexistent in equilibrium with the same composition.
Further, the LN single crystal as grown from this congruent composition, has a multi-domain structure. Accordingly, treatment for single domain used to be applied in such a manner that the crystal after growing was heated to a temperature of at least about 1,150.degree. C. i.e. the Curie point, and a voltage was applied in the Z axis direction of the crystal in the heated state to align the domain in one direction, followed by cooling the crystal. The single domain-treated crystal was processed into a predetermined size and then used for various optical applications.
As a single crystal to be used for an optical laser device for writing a three dimensional hologram in the single crystal by means of a visible light laser beam, the LN single crystal with a congruent composition having no transition metal incorporated, is transparent and of a high quality with a uniform crystal composition without crystal defects such as striations. However, the hologram diffraction efficiency is extremely low, and there was a problem that it was difficult to write an application hologram in the crystal.
Therefore, a LN single crystal having the hologram diffraction efficiency improved by an addition of a transition metal such as Fe to the LN single crystal, has been used for a research. However, with a crystal grown by an addition of iron in a substantial amount to the congruent composition, crystal defects such as striations are likely to be taken into the crystal, and such defects will bring about optical non-uniformity or non-uniform scattering of the laser beam, whereby there has been a problem that the hologram diffraction efficiency substantially varies depending upon the portion of the crystal, and it has been difficult to obtain a constant hologram diffraction characteristic.
Further, a coupling coefficient is used as a performance index for comparing the crystal materials showing such photo-refractive effects. In the case of Fe-doped LN single crystals so far known, the coupling coefficients are reportedly from about 4 to 12 cm.sup.-1 (for examples Optical Crystals, p.200, edited by Shintaro Miyazawa, published by Baifukan), which are not necessarily sufficient for optical applications.
Further, from the viewpoint of preparation of a crystal, a transition metal segregates in the crystal. Accordingly, it is difficult to grow a crystal having such a transition metal uniformly incorporated, and macroscopic crystal defects which cause light scattering, are usually contained in a substantial amount in a LN single crystal with a Fe-doped congruent composition. Thus, a product having a quality sufficient for an optical element has not been obtained.
Furthermore, a noise which is believed to be caused by random light scattering due to non-uniformity of the Fe-doped LN single crystal, has been regarded as a problem which prevents improvement of the recording density of the hologram. Further, with a crystal having iron incorporated into a congruent composition there has been a problem that it takes too long a time for the hologram diffraction efficiency to reach a constant level (the response time for writing a hologram). This results in difficulty in that the writing speed is too slow for application to a hologram-application element.
Recently, the present inventors have found it possible to improve the hologram diffraction efficiency without adding iron, by bringing the composition of the LN single crystal close to the stoichiometrical composition. Even in this case, the problem that the response time for writing a hologram is long, has not been solved.